The invention relates to an optical beam-splitting element having a first surface which is provided with a grating for generating a sub-beam having a second direction from a radiation beam incident on the first surface and having a first direction. The invention also relates to a radiation source unit comprising such an element and to an optical scanning unit comprising such a radiation source unit.
The direction of a beam is defined as the direction of the chief ray of the beam.
An element of this type is known from European Patent Application no. 0 583 036 in which it has been described as a component of an optical scanning unit. A radiation source in the scanning unit supplies a radiation beam which is focused by an objective lens to a scanning spot on an optical record carrier. The beam-splitting element, which is traversed by the radiation beam from a second surface to a first surface, is arranged between the radiation source and the objective lens. The radiation reflected by the record carrier traverses the same path as the radiation beam from the radiation source and forms the above-mentioned beam having a first direction and being incident on the beam-splitting element. A grating on the first surface of the beam-splitting element splits a part of the radiation of the incident beam in the form of a sub-beam having a second direction. The sub-beam converges on a detection system arranged next to the radiation source, which detection system generates, inter alia a signal from the incident radiation, which signal represents the stored information.
A drawback of the known beam-splitting element is the small angle between said first and second directions, which necessitates a relatively small distance between the radiation source and the detection system. If the radiation source and the detection system are two separate components, it is difficult to arrange them in the scanning unit at such a short distance from each other.